Effects of microstructural inhomogeneity on stress corrosion cracking behavior of Al–Zn–Mg–Cu alloys

Al–Zn–Mg–Cu thick plates are essential load-bearing materials for large-scale aerospace structures. However, microstructural inhomogeneity along thickness direction results in significant variations in sensitivity to stress corrosion cracking (SCC), which presents challenges for their widespread engineering applications. In this work, SCC behavior of surface, quarter (T/4) and middle (T/2) layers of Al–Zn–Mg–Cu thick plates were systematically investigated using double cantilever beam tests, electrochemical measurements, and multi-scale microstructural characterization. Results show that surface layer demonstrates superior SCC resistance, whereas T/4 layer exhibits the highest SCC sensitivity. In the surface layer, discrete distribution of grain boundary precipitates inhibits the penetration efficiency of Cl − . Furthermore, crack propagates through repeated cycles of crack arrest and re-nucleation, which increases the energy consumption of crack propagation. Conversely, in T/4 layer, segregation of Zn and Mg reduces the strength of grain boundaries, while the elevated dislocation density induces localized stress concentration, thereby facilitating rapid crack propagation. These findings provide insights into controlling SCC in alloys through regulation of grain and grain boundary characteristics.